Anika Hazra
Science Writer
When I Was a Scientist...
Prior to pursuing a career in science journalism, I was an ecologist. For my Master's research, I studied the influence of restored forest conditions on the growth of the Bullhorn acacia (Vachellia cornigera) and its occupancy by two species of ants, Pseudomyrmex ferrugineus and Pseudomyrmex gracilis. There were two major questions I wanted to answer with my research:
1) How do the conditions of an experimentally-restored tropical forest affect the establishment and growth of V. cornigera, a species that is found in the restored forest but was not planted there?
2) What environmental factors affect which species of ant occupies a V. cornigera individual, where environmental factors could refer to the restored forest or just the acacia individual that ants live on?
Before we get into the results of my research, let's get into what exactly an "experimentally-restored forest" is and why I wanted to study its conditions.
Experimental Forest Restoration
Figure 1. Satellite image of study site in Los Tuxtlas, Mexico with all 24 forest plots outlined in a yellow box; photo was taken in 2014, eight years after seedlngs were planted. Image provided by Google Earth.
Figure 2. Set-up of experimental restoration plots with treatment types (C = Control, A= Animal-dispersed species, W = Wind-dispersed species). Figure from De la Pena-Domene et al. (2013).
Restoration ecology is a scientific field in which researchers attempt to determine if restoring habitat (whether it happens naturally or if people actively get involved) can actually bring back a healthy, functioning ecosystem that supports a diversity of wildlife, and which restoration methods can most effectively accomplish this. Experimental restoration aims to measure the effectiveness of specific methods in a controlled environment.
My research was conducted at a tropical forest restoration site in Los Tuxtlas, Mexico (Figure 1). This site consists of 24 experimentally-restored forest plots embedded within active cattle pasture. The plots consist of three treatments: eight were planted with seedlings of tree species that are wind-dispersed, eight were planted with species that are animal-dispersed and eight were left to regrow naturally (unplanted) (Figure 2). All the plots are fenced to prevent cattle grazing.
This restoration site was established in 2006 and the young forest that is currently growing there is representative of habitat in the early stages of restoration. The site existed long before I showed up for grad school, and a lot of really interesting research has happened and continues to happen at this study site. Check out this article to find out what we've learned so far from the research projects that have taken place at these forests plots.
One commonly-used method for determining if a restored habitat has agreeable conditions and is functioning the way we want it to is by studying how certain species, which we call bioindicators, respond to the habitat.
Establishment Patterns of V. cornigera in Restored Forest
Vachellia cornigera (Figure 3) is a disturbance-tolerant pioneer species. This means that it is one of the first plant species to thrive in degraded or newly restored forest habitat, whether it is planted there or not. In the case of my study site, this species was not planted, so any individual growing at the site had its seeds dispersed to the site from somewhere else. Birds are the primary seed dispersers for V. cornigera; they eat the fruits of these trees and poop out the seeds some time later, usually after they've moved to a different area. Vachellia cornigera--being a pioneer species--should do well in our study site, but its establishment and growth may be affected by the different planting treatments and the overall conditions of the site, among other things.
I wanted to know if the following factors affect V. cornigera:
1) Restoration treatment
2) Location relative to each plot (inside the forest plot, just outside the plot or right at the edge)
3) Forest canopy cover
What I found is that restoration treatment did affect the number of V. cornigera individuals growing in any particular forest plot, where on average the greatest number of individuals were found in the "animal" plots (those that were planted with animal-dispersed tree species) and the fewest were found in the control plots (those that were unplanted). Location also had an effect on the abundance of V. cornigera individuals, where on average the greatest number of individuals were found growing at the edge of the plots, while the fewest were found growing inside the plots. This makes sense because pioneer species need a lot of light to grow, which they won't get in the denser parts of a forest. In addition to this, location also affected the height distribution of the trees, where the tallest ones were growing at the edge and the smallest were growing just outside the fenced plots. Oddly enough, forest canopy cover, measured using satellite imagery of the study site, did not have a direct relationship with V. cornigera abundance or height.
We now know that the restored forests' conditions are influencing the growth patterns of V. cornigera, but what about the ant colonies that live on these trees?
Figure 3. Photo of V. cornigera. Source: David Stang, Wikimedia Commons
The Occupancy State of V. cornigera Individuals
The reason why any ants live on V. cornigera in the first place is because this tree species has evolved a mutualistic relationship with certain species of ants, such as Pseudomyrmex ferrugineus (Figure 3). In this relationship, a colony of P. ferrugineus ants lives inside the hollow thorns of a V. cornigera tree, feeding on nectar secreted by glands on the tree and small, yellow bead-like structures growing at the tips of leaves, called Beltian bodies, that are rich in proteins and lipids. In exchange for shelter and food, the ant colony aggressively defends their host tree from anything that wants to eat it, as well as other plants growing too close to it.
Having all these resources readily available to mutualistic ants, however, makes V. cornigera vulnerable to other ant species that will take advantage of it. One such species is Pseudomyrmex gracilis (Figure 4), which is an exploiter, or opportunist. You can read about the difference between opportunists, cheaters and straight up parasites here. As an exploiter, P. gracilis colonies will move into unoccupied V. cornigera trees and feed on their nectar, without defending the them.
Figure 4. Front, lateral and dorsal views of Pseudomyrmex ferrugineus. Source: Antweb.org
Figure 5. Front, lateral and dorsal views of Pseudomyrmex gracilis. Source: Antweb.org
I wanted to know if the conditions of a V. cornigera individual, or of the restored habitat in general, affect the type of ant species that occupies the individual, where each individual was found to be in only one of three occupancy states: occupied by P. ferrugineus, occupied by P. gracilis or not occupied by any ants.
I focused on the following factors:
1) Restoration treatment
2) Location of V. cornigera individual relative to forest plot
3) Height of V. cornigera individual
I determined the effect of occupancy state on the condition of V. cornigera by estimating the level of herbivory damage incurred on each tree. I did this to observe how the conditions of V. cornigera can both influence and be influenced by the ant species that occupies it.
Restoration treatment, unsurprisingly, did not affect the occupancy state of V. cornigera--however, location did. The greatest proportion of individuals that were occupied by either ant species was found at the edge of forest plots. Most of the trees found in the interior of forest plots were not occupied by ants, so was a moderate proportion of trees found just outside the plots. A moderate number of trees outside the plots were occupied by P. gracilis. The height of V. cornigera had a clear effect on occupancy state, where the tallest individuals (1-5 m) were occupied by P. ferrugineus, the mid-sized individuals (30-60 cm) were occupied by P. gracilis and the smallest (30-45 cm) were not occupied.
The occupancy state of a V. cornigera individual influenced the level of herbivory damage it incurred, but not exactly how I expected it would. Trees that were occupied by P. ferrugineus had the lowest level of damage (<1%, on average), while those occupied by P. gracilis had closer to 50% damage on average and those not occupied at all had slightly more damage.
I think the most obvious conclusion that can be drawn from all of these results is that edge habitat is extremely important in our restored forest study site. The edge of forest plots is where most of the V. cornigera indviduals are, it's where the tallest individuals are and, because P. ferrugineus prefers larger host trees, it's where this mutualistic ant species is found as well.
My Master's Thesis (available on INDIGO):
http://hdl.handle.net/10027/22668